Patent GB-512796-A (GB)
Title: Improvements in or relating to magnetic direction indicators
IPC: G01C17/30
Filed: 1938-03-15
Abstract: 512,796. Magnetic tests. SIEMENS APPARATE UND MASCHINEN GES. March 15, 1938, Nos. 8035 and 8036. Convention dates, March 15,1937, and Sept. 21, 1937. [Class 37] A magnetic direction indicator comprises one or more bars of material of high permeability pivoted about a transverse axis, one or more windings carried by the bar or bars, means for producing an alternating or pulsating flux in the bar or bars of insufficient value to cause saturation even when the horizontal component of the earths field is superposed thereon, whereby the impedance of the winding or windings varies with the direction of the bar relatively to the earth's field, and apparatus for measuring the impedance. A winding 2, Fig. 1, encloses the central part of a bar 1 of permalloy or other high permeability material which is pivoted in a fork 4 so that it sets itself horizontally, or the horizontal setting may be effected by stabilising devices. The loop 4 is fixed to a spindle 6 which can be turned by a worm-wheel 7 and worm 8 driven from a course indicator 9 through a flexible shaft 10 so as to adjust the bar 1 in azimuth. The ends of the winding 2 are connected to slip-rings by which it is connected to an alternating current source 16 through an ammeter 15. The component of the earth's field in the longitudinal direction of the bar produces a direct magnetic flux. which is superposed on the alternating flux and thus varies the impedance of the coil 2 and therefore the value of the current flowing. In order to differentiate between angular positions of the bar 1 on either side of the zero or East-West position, an additional winding connected to a D.C. source may be provided, the flux from which is added to or subtracted from that due to the earth's field according as the angular position is on one side or the other of the zero position. The direct current may be obtained from a separate D.C. source or through rectifiers from an A.C. source which may be the source 16. The sensitivity may be increased by adjusting the value of the D.C. so that the working point is displaced to the steep part of the magnetisation curve. The sensitivity may be further increased by rectifying in the circuit containing the A.C. source 16 and feeding the additional winding with a portion or the whole of the rectified current. Alternatively the ammeter 15 or a motor &c. connected in its place may control a potentiometer, bolometer, or electromagnet which controls the D.C. fed to the additional winding. The coil 2 may alternatively be supplied with pulsating direct current which may be derived from a D.C. source through a mechanical interrupter, or from an A.C. source through a vibrating or other half-wave rectifier. The variation of impedance of the coil 2 may alternatively be measured by a bridge arrangement, or by measuring the voltage across the winding. The change in magnitude of the alternating flux in the bar 1 caused by the earth's field may alternatively be measured by means of a secondary coil connected to a voltmeter. The current that would flow in this coil in the zero position of the bar 1 may be compensated by means of impedances and a current source. Two bars 1, each provided with a coil 2, may form two arms of a bridge, the other two arms being formed by resistances 29, 30, Fig. 9. In another modification, Fig 10, the windings 2 are connected to an instrument 33 having two windings. The polarity of the periodically varying flux is opposite in the two bars 1, so that the component of the earth's field is additive in one bar and subtractive in the other, and the connections are such that in the zero position of the bars that zero indication is obtained on the instrument 33. The windings 2 may be in oscillatory circuits the constants of which depend on the flux in the bars due to the earth's field. Amplifiers may be employed with low sensitivity instruments. The bar 1 may have a constriction at the part carrying the winding 2 and the cross-section at the ends of the bar may be considerably larger than at the remaining part. The windings may be on a single carrier or on separate carriers. The periodically varying magnetic flux in the bar 1 may alternatively be produced by a rotating or oscillating permanent or electromagnet or, as shown in Fig. 13, by rotating magnetic members 112, 113 between bar magnet 111 and the permalloy bar 101. In a modification, Fig. 15, an electromagnet 115 is excited from an A.C. winding 117 and may also have a D.C. winding 119, or a single winding fed by periodically varying direct current may be used. The members 112, 113, Fig. 13, may be replaced by a disc rotating between and parallel to the magnet and bar and formed with recesses or slots. The magnetic source may be horizontally displaced. The apparatus may be used as a steering instrument as well as or instead of an indicating instrument.